Given the relatively low fertility and mortality levels prevailing in the United States, the future growth and structure of the population will be increasingly affected by immigration. This immigration effect stems from two mechanisms: net migration and the fertility of the foreign-born. While considerable attention has been devoted to the former, immigrants' fertility contribution remains poorly understood. This is problematic because as the immigrant population grows, national fertility patterns will by definition be increasingly determined by their behavior. Between 1990 and 2010 the share of U.S. children born to immigrant women increased from 16 to 23%. Today, without the fertility of immigrants there would be nearly one million fewer U.S. births annually. This impact highlights the need for systematic analyses of immigrant fertility, including the development of demographic techniques that can clearly separate the population growth contribution of immigration from its fertility effet. Particularly problematic is the lack of understanding of the interaction between immigration and fertility timing. Standard demographic techniques generally assume the two components are disconnected. However, studies have highlighted that migration can significantly distort the age-pattern of childbearing. Failure to account for the impact of immigration on the timing of births results in a substantial overestimation of period levels of immigrant fertility. Since age-specific fertility rates are a key component of population projections, this overestimation distorts projections of the size, age, and ethno-racial structure of national populations. Thus, our specifi objectives are to 1) Measure the contribution of immigrants to U.S. fertility for the period 2000-2010 and its variation over time and by ethnic group, distinguishing between Hispanics, Asians, and non-Hispanic blacks and whites; 2) Separate, within racial/ethnic categories, national origin groups and measure their fertility contribution elaborating on the role that changing immigrant national origin has on U.S. fertility; 3) Assess the applicability of standard fertility measures t the immigrant experience and its variation over the period and across ethno-racial groups. Specifically, we will assess the role of tempo distortions resulting from immigration in affecting standard period fertility measures; 4) Develop and test fertility measures that explicitly take int account the dynamics of migration to measure the fertility of immigrants over time and across ethno-racial groups. Specifically, this implies recognizing that births to immigrant women occur before and after migration; separating the two components is necessary to measure the fertility- specific contribution of immigration; and 5) Incorporate these measures into population projection methods and assess their impact on predicted population change for 2000-2010. We will also generate a new set of population projections through 2050 and compare them to Census estimates.